2,5-Furandicarboxylic acid (“FDCA”) is a dicarboxylic acid for which the commercial interest has grown recently. The diacid or its diester has been found to be particularly interesting for the preparation of poly(alkylene-2,5-furandicarboxylate), in which preparation FDCA is condensed with an alkylene glycol, such as monoethylene glycol (MEG). The polymer obtained from the polymerization of FDCA or its diester and MEG, polyethylene-2,5-furandicarboxylate (PEF), can be used as alternative for polyethylene terephthalate (PET) or, due to its properties, can be used in areas where PET cannot be used. As one of the advantages of using FDCA it is considered that FDCA is obtainable from sustainable sources. In WO 2011/043661 and U.S. Pat. No. 8,519,167 methods are described wherein 5-hydroxymethyl furfural and derivatives thereof are converted into FDCA and esters thereof. As explained in these patent documents, the starting material 5-hydroxymethylfurfural and derivatives thereof can be obtained from carbohydrate-containing sources, such as fructose, glucose, sucrose, starch, cellulose etc.
US 2012/0302768 discloses that the oxidation process of 5-hydroxymethylfurfural and derivatives thereof leads to the formation of a mixture of 2,5-furan-dicarboxylic acid, 2-formyl-furan-5-carboxylic acid and, optionally, some other furan derivatives, such as alkyl esters of 2-formyl-furan-5-carboxylic acid. Although it is stated that purified 2,5-furandicarboxylic acid can be obtained by washing, it was found that also the purified product still contained an amount of 2-formyl-furan-5-carboxylic acid. It is acknowledged in US 2012/0302768 that significant concentrations of mono-functional molecules like 2-formyl-furan-5-carboxylic acid in the 2,5-furan-dicarboxylic acid product are particularly detrimental to polymerization processes as they may act as chain terminators during a polyester condensation reaction.
It has been found that washing does not yield pure product. It is believed that 2-formyl-furan-5-carboxylic acid is included in the crystals of 2,5-furan-dicarboxylic acid whereby the purification by washing becomes unfeasible. Purification of the corresponding ester products was found to be easier.
The need for a method for the production and purification of esterification products of FDCA has been acknowledged in WO 2013/191938. In order to meet this need WO 2013/191938 describes a process wherein a liquid reaction mixture comprising FDCA, an alcohol, water, the monoester and the diester of FDCA, is subjected to esterification. The esterification product is vaporized such that a vapor comprising the mono- and diester of FDCA, unreacted alcohol and water, is removed from the reaction mixture and passed to a rectification zone. In the rectification zone at least a portion of the monoester of FDCA is condensed and the liquid monoester is subsequently contacted with the reaction mixture. The diester of FDCA is continuously discharged for the rectification zone, together with water, unreacted alcohol and by-products.
A drawback of this process resides in that the vaporized esterification product removes not only water, but also the alcohol and the mono- and diester of FDCA. Therefore, the vapor stream is large, which requires a high amount of energy. Further, due to the range of boiling points of the components of the vapor, i.e. water, alcohol, monoester and diester of FDCA, the rectification is complicated. Moreover, by the removal of the monoester together with the alcohol and water the production of the desired diester of FDCA is sub-optimal. Hence, there is a need for a process for the preparation of alkyl esters of FDCA, wherein the separation of the various components in the reaction mixture is facilitated and the yield of the esterification reaction is improved compared to this known process.